Many tumor types have been shown to exhibit an absolute requirement for methionine in vitro in order to proliferate. Normal cells and tissues can utilize homocysteine in place of methionine in order to proliferate. Methionine dependence causes methionine-dependent tumor cells to synchronously arrest in the late-S/G2 phase of the cell cycle. The selective synchronization of tumor cells by methionine deprivation allows the tumor cells to be selectively killed by antitumor drugs such as vincristine when the cell cycle block is alleviated with methionine. In order to bring methionine-dependent chemotherapy closer to the clinic, we propose here to develop assays of methionine dependence with actual tumor specimens put into histoculture. These assays will be based on biochemical parameters, which involve measurements of S-adenosylmethionine to S-adenosylhomocysteine ratios, and on cell cycle parameters which involve measuring arrest in the cell-cycle as late S/G2 in methionine-free homocysteine-containing medium. These assays will be compared in Phase I to their ability to predict in vitro susceptibility of the tumors to methioninedependent chemotherapy based on methionine depletion and subsequent methionine repletion with chemotherapeutic drugs. In Phase II we will compare the assays ability to predict sensitivity to methioninase-based therapy in vivo with the purified methioninase currently produced in our laboratory.